Capillary zone electrophoresis ("CZE") in small capillaries (less than or equal to 75.mu.) was first demonstrated by Jorgenson and Lukacs, and has proven useful as an efficient method for the separation of small solutes. J. Chromatog., 218 (1981), page 209; Anal. Chem., 53 (1981), page 1298. The separation process relies upon an electroosmosis effect generally described as the flow of a liquid in contact with a solid surface under the influence of a tangentially applied electric field. Attractive factors for electrophoretic separations by capillary zone electrophoresis are the small sample sizes, little or no sample pretreatment, and the potential for quantification and recovery of biologically active samples.
For example, U.S. Pat. No. 4,675,300, inventors Zare et al., issued June 23, 1987 describes theories and equipment for electrokinetic separation processes employing a laser-excited fluorescence detector. The system described by Zare et al. includes a fused silica capillary with a 75.mu. inside diameter.
Unfortunately, one of the single greatest disadvantages of capillary zone electrophoresis lies when attempts are made to separate macromolecules such as proteins. Separations of macromolecules by CZE leads to untoward interactions of the biopolymers with the silica capillary wall.
Jorgensen et al. had noted that separation of model proteins, such as cytochrome, lysozyme and ribonuclease A, in untreated fused silica capillaries with a phosphate buffer at pH 7 was accompanied by strong tailing, and suggested this might be caused by Coulombic interactions of the positively charged proteins and the negatively charged capillary wall. Jorgensen et al., Science, 222 (1983) page 266.
Lauer et al., Analytical Chemistry, 58 (1986), page 166, has reported that the Coulombic repulsion between proteins and the capillary wall of silica capillaries can overcome adsorption tendencies of the proteins with the capillary wall. They demonstrated separations of model proteins (ranging in molecular weight from 13,000 to 77,000) by varying the solution pH relative to the isoelectric point (pI) of the proteins to change their net charge. However, disadvantages of this approach are that silica begins to dissolve above pH 7, which shortens column life and degrades performance, only proteins with pI's less than the buffer pH can be analyzed, which drastically reduces the range of useful analysis, and interactions which are not Coulombic may still occur even with proteins bearing a net negative charge due to the complexity of protein composition and structure.
Another approach to the problem of biopolymer, or protein, interactions has been to increase ionic strength. It has been demonstrated that this concept works in principle, but heating is also increased as ionic strength is increased. This heating tends to degrade the efficiency of separation.
Yet another approach to the problem of undesirable protein interactions with the capillary wall has been to coat the electrophoresis tube with a mono-molecular layer of non-crosslinked polymer. Thus, U.S. Pat. No. 4,680,201, inventor Hjerten, issued July 14, 1987 describes a method for preparing a thin-wall, narrow-bore capillary tube for electrophoretic separations by use of a bifunctional compound in which one group reacts specifically with the glass wall and the other with a monomer taking part in a polymerization process. This procedure results in a polymer coating, such as polyacrylamide coating, and is suggested for use in coating other polymers, such as poly(vinyl alcohol) and poly(vinylpyrrolidone). However, this method and capillary tube treatment tends to destroy the electroosmotic flow, and efficiencies are still rather low. These rather low efficiencies suggest that undesirable protein-wall interactions are still occurring.